For purposes of this application, the term "boronic" shall mean the element boron or a composition containing boron.
Many composite materials are being reinforced with high strength, high modulus filaments such as boron or silicon carbide. The composite matrix materials provide high strength and stiffness while at the same time give reduced component weight.
U.S. Pat. No. 4,068,037 to DeBolt et al and assigned to the same assignee as this invention discloses means for making silicon carbide filament. In accordance with the DeBolt et al patent, a carbon filament is drawn from a supply reel and passed through a reactor chamber comprised generally of a closed tubular cylinder having oppositely disposed closed ends. In each of the closed ends, there is a central aperture which allows the carbon filament to pass into and out of the reactor on an uninterrupted basis. A mercury contact in each aperture allows that portion of the filament which is within the reactor to be raised in temperature by resistance heating. A number of ports in the sidewalls of the reactor tube allow chemicals to be injected into and withdrawn from the reactor chamber. Typically, a mixture of hydrogen and silanes are fed into the tubular cylinder to deposit silicon carbide. When the silanes come in contact with the heated filament core, a chemical vapor deposition process takes place, and silicon carbide is deposited on the core.
DeBolt et al further teaches that it is advantageous to apply a surface layer of carbon-rich silicon carbide to the silicon carbide coating during the deposition process. The carbon-rich layer was shown to both improve tensile strength and decrease sensitivity to surface abrasion. The layer was approximately one micron thick and had a chemical composition which varied from pure carbon at the outer surface to silicon carbide at a depth of one micron. This carbon-rich coating both enhanced the strength of the filament and at the same time made the filaments easier to handle by the operator.
The carbon-rich filament of DeBolt et al does exhibit certain deficiencies. For example, when the filaments are fabricated into an aluminum composite, they do not wet easily in molten aluminum. Wetting can be accomplished at very high temperatures, but this degrades the strength of the filaments, presumably by chemical reaction between the aluminum and the carbonaceous surface. Aluminum alloys containing magnesium, nickel or titanium wet the filament at lower temperatures, but degrade their strength by chemical attack. In aluminum composites fabricated at lower temperatures, for example, by diffusion bonding, there is poor bonding between the filament and the matrix. In composites where the matrix material is titanium, the properties of the composite suffer because of mutually adverse interaction between the filament and the matrix. In matrix materials such as epoxy resins, it has also been difficult to provide good bonding between the carbonaceous surface and the resin.
With our invention, these problems are corrected. We apply boronic refractory material coatings such as boron or boron carbide to the silicon carbide filament preferably by chemical deposition. The boronic refractory material coatings provide protection from chemical attack of the filament by matrix materials during processing, fabrication and high temperature use. A thin outer boronic layer in the order of 1 micron or less may be used to enhance wetting and bonding of the filament to the matrix material. Heavier coatings would have the same effect, but is not preferred because heavier coatings take longer to produce. Heavier coatings are used to protect the silicon carbide from attack by the matrix material when the silicon carbide is used to make a composite material. Tests show that it does not seriously effect the strength of the underlying silicon carbide filament. Addition of the boronic refractory material coatings may be used to enhance the strength of the resulting filament.
The boronic refractory material coatings may be applied either directly to the silicon carbide or to a silicon carbide filament which has a carbonaceous outer skin. The methods and conditions under which the coatings are applied are unique to this invention.